(1) Field of the Invention
The present invention relates to semiconductor devices for use in millimeter wave communications or power switching applications.
(2) Description of Related Art
Group III nitride semiconductors typified by gallium nitride (GaN) and serving as a mixed crystal material represented by the general formula (InxAl1-x)yGa1-yN (where 0≦x≦1 and 0≦y≦1), have been expected to be applied not only to light-emitting devices for emitting visible or ultraviolet light by utilizing their physical features, i.e., a wide band gap, and a direct transition band structure, but also to electronic devices by utilizing their features, i.e., high breakdown voltage and high saturated electron velocity. Particularly, a heterojunction field effect transistor (hereinafter abbreviated as HFET) which employs 2 dimensional electron gas (hereinafter abbreviated as 2DEG) occurring at the interface between epitaxially grown AlxGa1-xN and GaN has excellent properties of high current density and high breakdown voltage. The high current density results from the high sheet carrier density of the 2DEG, and the high breakdown voltage results from the wide energy band gap of AlGaN and GaN. As a result, HFETs have been expected as high-power and high-frequency devices and have been earnestly studied and developed.
For a semiconductor device made of a Group III nitride semiconductor, as described in, for example, A. V. Vertiatchikh, L. F. Eastman, W. J. Schaff and T. Prunty, “Effect of surface passivation of AlGaN/GaN heterostructure field-effect transistor”, Electronics Letters vol. 38, pp. 388-389 (2002), a silicon nitride film is typically used as a surface protection film (SiN passivation).
Furthermore, in addition to the SiN passivation, a method in which an epitaxially grown aluminum nitride (AlN) layer is used as part of a passivation film has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 2006-279032).
When the surface of a known gallium nitride based HFET is covered with a passivation film made of silicon nitride (SiN), this prevents heat from being dissipated from the device surface. The reason for this is that SiN used as the passivation film has small thermal conductivity. In particular, for a high-power transistor, a large drain current flows in the vicinity of the device surface. This allows the device temperature to reach several hundred degrees. Such an increase in the device temperature causes degradation in device characteristics, such as a reduction in the drain current. This makes it essential that heat is efficiently dissipated from the device.
Furthermore, when, instead of the deposited film made of silicon nitride, an epitaxial layer made of aluminum nitride is used as a passivation film, the step of selectively removing a region of the AlN epitaxial layer in which an ohmic electrode is to be formed (hereinafter, referred to as “ohmic electrode formation region”) by dry etching or any other method is further required. In addition, when the AlN epitaxial layer is subjected to dry etching, etching damage is caused also to an ohmic electrode formation region, leading to an increase in contact resistance.